The present invention relates to a scattered X-ray correcting method, an X-ray computerized tomograph (CT), and a multi-channel X-ray sensor, and in particular, to an X-ray CT having a mechanism for correcting influence of scattered X-rays and a multi-channel X-ray sensor having a function to sense scattered rays.
In an X-ray CT, an X-ray beam emitted from an X-ray tube is collimated into a fan shape to be passed or penetrated through a subject so as to resultantly attain information of attenuation thereof. The attenuation information is attained by an X-ray sensor disposed behind the subject. While keeping the relative position of the X-ray beam with respect to the X-ray sensor, the beam emitter is turned about an axis of the subject. Attained information is processed by a computer of the tomograph to produce or reconstruct an X-ray tomogram.
To process images at a high speed, there is employed a multi-channel X-ray sensor in which a plurality of X-ray sensing elements are ordinarily arranged in a circular arc with an X-ray tube disposed at a focal point thereof.
In this connection, when the X-ray beam enters the subject, the beam is not only absorbed by textures or tissues to be thus attenuated but also subjected to scattering due to interaction with atoms constituting the tissues. As known, the X-ray scattering is classified into the Rayleigh scattering related to a phenomenon of interference and the Compton scattering not related thereto.
(1) Rayeigh scattering
When a photon of the X-ray passes a position in the neighborhood of an electron, the electron is caused to vibrate according to an electric field formed by the photon to resultantly absorb the photon such that the electron radiates a photon having the original oscillation frequency. In consequence, the scattered X-ray has a wavelength equal to that of the incident X-ray, leading to interference between the incident and scattered X-rays. Probability of occurrence of the Rayleigh scattering associated with the interference is smaller than that of the Compton scattering. However, the Rayleigh scattering is characterized by sharp scattering of particles toward the front side of scattering. In a case where X-ray having low energy is incident to a substance having a large atomic number, the scattered rays resultant from the Rayleigh scattering take a large part in the overall scattered rays.
(2) Compton scattering
When a photon of the X-ray collides with a free electron of a substance or an electron on an outer orbit of an atom weakly linked with a kernel thereof, the collided electron cannot absorb the entire energy of the photon and hence emits a portion of the energy in the form of a photon. As a result, the electron is emitted according to the remaining kinetic energy. This phenomenon is called Compton effect. In regard to the incident photon, the photon re-emitted by transforming the energy and the photon emitted due to the collision are called Scattered photon (X-ray) and Compton electron, respectively. When the energy of the incident X-ray becomes higher, the ratio of the Compton scattering is increased in the overall scattering.
(3) Influence of scattered X-ray on image
Each of the sensing elements measures attenuation of the X-ray in a portion of the subject on a line (measuring path) drawn between the focal point of the X-ray tube and the center of the sensing element. When there exist scattered X-rays from other portions of the subject, an error appears in measured values. The scattered X-rays incident to the element increase the output therefrom and hence X-ray attenuation through the portion of the subject in the measuring path is virtually decreased in the data resultant from measurement. When the error becomes greater, there occurs deterioration in resolution of a CT image produced from the measured data. Particularly problematic is reduction in low-contrast resolution called density resolution. Additionally, there clinically exists a rib archifact in which the CT value is virtually decreased for the inside of ribs and hence a dark zone resultantly appears in the obtained image or picture. Moreover, there possibly exists a case in which the CT value varies between various positions in the liver. Consequently, to attain an accurate X-ray tomogram, it is necessary to remove influence of scattered X-rays.
The known methods of removing influence of scattered X-rays are as follows.
(1) Scattered X-rays are prevented from entering an X-ray sensing surface of each channel of the X-ray sensor. PA1 (2) The output component caused by the pertinent scattered X-rays is subtracted from each channel output of the X-ray sensor. PA1 (a) It is necessary to execute processing to obtain scattering constants Ca, Cw, and Cs so as to estimate the content of scattered rays in the measured data, scattering correction processing for water phantom data according to the scattering constants Ca and Cw and other data, and scattering correction processing for raw data according to the scattering constants Ca and Cs and other data. This consequently increases the amount of processing and the load imposed on the computer. PA1 (b) Although the various processing of (a) may possibly remove the influence of scattered rays, there exist quite complicated relationships between occurrence and influence of scattered rays and some of such relationships are independent of the internal constitution of the subject. In consequence, the above correction method corresponding to the measured values (proportional to attenuation of rays) is not sufficient to completely correct influence of the scattered rays.
As a specific method of the procedure (1) above, there have been proposed provision of a grid parallel to the X-ray sensing surface of each channel (described in, for example, the JP-A-62-60539 and JP-A-4-336044 ) and arrangement of a filter in front of the sensing surface (described in, for example, JP-A-62-270137).
The procedure (2) is in general achieved as follows. Namely, the quantity of scattered X-rays is actually measured so as to subtract the obtained value from the output of each channel. There has been proposed a configuration in which a plurality of scattered X-ray sensors are disposed to sense scattered-X-ray in front of a main X-ray sensor (described in, for example, JP-A-63-305846, JP-A-63-38438, JP-A-63-40534, and JP-A-1(64)-62126). Moreover, there has been proposed a method in which the X-ray sensor includes only a main sensor such that a rod made of lead is arranged to absorb X-rays in front of each channel so as to remove only X-rays which serve as a signal in the actual measurement (as described in, for example, JP-A-62-261342).
(3) In addition, another specific method of the procedure (2) has been described in the JP-A-4-170942.
In this method, there are prepared scattered X-ray correction constants including an air correction constant calculated from a regression line y=ax+b associated with the total channel data value for a first rotary angle and a correction constant related to a water phantom. In the actual case, after all measured data values (for air calibration, phantom calibration, and subject measurement) are converted into the logarithmic expression and then are multiplied by correction coefficients after the logarithmic conversion. Namely, the correction is conducted after the measured data are converted into the logarithms expression.
According to the prior art described above, when the measurement is achieved using the procedure (1) to remove X-rays entering the sensor, the incident intensity of the signal X-rays is also lowered and hence the signal-to-noise (S/N) ratio is deteriorated. This accordingly leads to a problem of an unsatisfactory CT image. On the other hand, in association with the procedure (2) to actually measured scattered X-rays so as to remove the quantity thereof from the data, the method using a sensor dedicated to scattered X-rays is attended with the following problem.
The scattered X-ray sensor arranged in front of the main X-ray sensor increases thickness of the overall body of X-ray sensor, leading to difficulty in mounting the sensor for measurement. To solve the problem, there has been known a method in which a scattered X-ray sensor of a small size is disposed to be adjacent to the main X-ray sensor in a channel slicing direction at the central portion of the main X-ray sensor so as to measure the quantity of scattered X-rays in the central portion. The quantity of scattered X-rays in each channel in the peripheral portion is obtained through calculation according to an expansion function.
However, between the main sensor (usually having structure like that of an ionization chamber) and the scattered X-ray sensor (ordinarily having constitution like that of a solid state sensor), there appears a discrepancy in scattered X-ray sensitivity due to structural difference therebetween. Consequently, this configuration is not advantageous for practices.
Furthermore, the construction including the scattered X-ray sensor in addition to the main X-ray sensor requires the expensive sensor and sensing circuit to be duplicated. This considerably increases the cost thereof in any case where the scattered X-ray sensor is in front of or adjacent to the main sensor.
On the other hand, according to the procedure (2), in the method in which only the main X-ray sensor is used so as to employ a shielding device to remove signal X-rays when measuring scattered X-rays, a complex function is additionally required to operate the apparatus while keeping the positional accuracy of the shielding device. This inevitably leads to increase in the cost of the system.